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CHAPTER XXII 



IRON, COBALT, AND NICKEL 



JUDGING from the' atomic weights, and the forms of the higher oxides 

 of the elements already considered, it is easy to form an idea of 

 the seven groups of the periodic system. Such are, for instance, the 

 typical series Li, Be, B, C, N, O, F, or the third series, Na, Mg, Al, Si, 

 P, S, Cl. The seven usual types of oxides from R 2 O to R 2 O 7 correspond 

 with them (Chapter XV.) The position of the eighth group is quite 

 separate, and is determined by the fact that, as we have already seen, 

 in each group of metals having a greater atomic weight than potassium 

 a distinction ought to be ..made between the elements of the even and 

 uneven series* The series of even elements, commencing with a 

 strikingly alkaline element (potassium, rubidium, caesium), together with 

 the uneven series following it, and concluding with a haloid (chlorine, 

 bromine, iodine), forms a large period, the properties of whose members 

 repeat themselves in other similar periods. The elements of the eighth 

 group are situated between the elements of the even series and the ele- 

 ments of the uneven series following them. And for this reason elements 

 of the eighth group are found in the middle of each large period. The 

 properties of the elements belonging to it, in many respects independent 

 and striking, are shown with typical clearness in the case of iron, the 

 well-known representative of this group. 



Iron is one of those elements which are not only widely diffused in 

 the crust of the earth, but also throughout the entire universe. Its- 

 oxides and their various compounds are found in the most diverse 

 portions of the earth's crust ; but here iron is always found combined 

 with some other element. Iron is not found on the earth's surface in 

 a free state, because it easily oxidises under the action of air. It is 

 occasionally found in the native state in meteorites, or aerolites, which 

 fall upon the earth. 



Meteoric iron is formed outside the earth. 1 Meteorites are fragments 

 "which are carried round the sun in orbits, and fall upon the earth 



The composition of meteoric iron is variable. It generally contains nickel, phos- 

 phorus, carbon, &c* The echreibersite of meteoric stones contains Fe4Ni 2 P. 



